1. Field of the Invention
The present invention relates to a method for setting the rotational speed of register rollers for transporting a recording medium, such as a transfer sheet, at a predetermined timing, and an image forming apparatus, such as a copying machine, a printer, or a facsimile device, capable of using the method.
2. Description of the Related Art
Conventionally, many image forming apparatuses of this type which control a rotation start timing of register rollers to control the distance from an end of a recording medium to an image write position are known. Such image forming apparatuses are disclosed in, for example, Japanese Patent No. 2,709,218, and Japanese Unexamined Patent Application Publication Nos. 2000-159395 and 5-127484.
In general, in an apparatus, such as a monochromatic image forming apparatus, for directly transferring an image onto a recording medium from a single image carrier, the speed of movement of the surfaces of register rollers (hereinafter referred to as “register linear speed”) is set slightly greater than the speed of movement of the surface of the image carrier. When the register linear speed is set slightly greater, the recording medium between the register rollers and a transfer area becomes flexed while the recording medium is passing through the transfer area. This makes it difficult for the register rollers to affect the recording medium in the transfer area. Consequently, since the register rollers do not easily affect the recording medium during transfer, a reduction in image quality caused by the effects of the register rollers can be restricted.
There is a strong demand for a color image forming apparatus to have higher printing speed. Therefore, in recent years, what is called a tandem color image forming apparatus using a direct transfer method is mainstream. The tandem color image forming apparatus is such that transfer areas of a plurality of image carriers are disposed above the transportation path of a recording medium. In the tandem color image forming apparatus, the recording medium is transported by being carried by a surface of a sheet transporting belt (member for transporting the recording medium). Then, toner images on the respective image carriers are successively transferred on the recording medium transported by the sheet transporting belt so as to be superimposed upon each other, as a result of which a color image is formed on the recording medium.
As in the monochromatic image forming apparatus, in such a tandem color image forming apparatus, when the register linear speed is set slightly greater than the speed of movement of the surface of the sheet transporting belt (belt movement speed), color misalignment occurs.
Hereunder, taking a tandem color image forming apparatus comprising four image carriers as an example, the reason why color misalignment occurs when the register linear speed is set greater than the speed of movement of the sheet transporting belt will be given. In the explanation below, the image carriers are called a first image carrier, a second image carrier, a third image carrier, and a fourth image carrier with increasing distance from the register rollers.
The recording medium transported by the register rollers is attracted to the sheet transporting belt, and is transported to the transfer area of each image carrier by the movement of the surface of the sheet transporting belt. Ideally, the recording medium and the sheet transporting belt are in complete contact with each other, and are not affected by disturbances at all. In this case, almost no color misalignment occurs. Actually, however, sliding of about a few μm to a few hundred μm occurs between the recording medium and the sheet transporting belt due to a disturbance. In addition, the belt movement speed may change due to a change in a load exerted upon the sheet transporting belt by a disturbance. Disturbances causing such sliding or changes in the belt movement speed are primarily caused by the effects of the register rollers driven at a linear speed that does not match the speed of movement of the sheet transporting belt.
More specifically, when the recording medium is transported by the register rollers driven at a register linear speed Vr, the recording medium is attracted to the sheet transporting belt driven at a belt movement speed Vt (Vt<Vr). Here, the speed of movement of the portion of the recording medium attracted to the sheet transporting belt is Vta (Vt<Vta<Vr) instead of the belt movement speed Vt. At this movement speed, an end of the recording medium moves into the transfer area of the first image carrier. Thereafter, as the recording medium is transported, the area of close contact between the recording medium and the sheet transporting belt increases, so that the speed of movement of the recording medium is controlled by the sheet transporting belt rather than the register rollers. By the time the end of the recording medium reaches the transfer area of the fourth carrier, the speed of movement of the recording medium is substantially equal to the movement speed Vt of the sheet transporting belt.
In a tandem color image forming apparatus, when the movement speeds of the recording medium passing the transfer areas of the respective image carriers are not the same, color misalignment occurs. In the aforementioned example, the movement speed of the recording medium is Vta when it passes the transfer area of the first image carrier. Thereafter, the movement speed of the recording medium is gradually reduced from Vta and becomes Vt when it passes the transfer area of the fourth image carrier. Therefore, the tone images of respective colors transferred from the respective image carriers are transferred to locations that are displaced from each other in correspondence with the speed differences, thereby resulting in color misalignment.
Based on the foregoing discussion, it can be understood that in order to form a high-quality image without color misalignment by a tandem color image forming apparatus, the register linear speed and the movement speed of the sheet transporting belt need to be set equal to each other with high precision. A method for performing feedback control on the driving of the sheet transporting belt in order to set its target value at the register linear speed is effective in making the speeds equal to each other with high precision. According to this method, feedback control makes it possible to absorb a slight difference between the movement speed of the sheet transporting belt and the register linear speed caused by, for example, the imprecision in manufacturing the sheet transporting belt itself, or the imprecision in the parts or the mounting of the parts of a drive system of the sheet transporting belt. Therefore, even if any of the aforementioned imprecisions occur slightly, the movement speed of the sheet transporting belt and the register linear speed can be set at values close enough to sufficiently restrict color misalignment.
Research conducted by the inventor showed that, if the operating environment changes, it is difficult to set the movement speed of the sheet transporting belt and the register linear speed at sufficiently close values even if feedback control is carried out. The reason is as follows.
If the operating environment is a constant environment (such as normal temperature and normal humidity), a range of a difference between the movement speed of the sheet transporting belt and the register linear speed capable of being absorbed by feedback control is relatively wide. However, if the operating environment deviates from the constant environment, this range is narrowed. Therefore, even if the movement speed of the sheet transporting belt and the register linear speed are set at sufficiently close values by feedback control when the operating environment is constant, it is difficult to set them at sufficiently close values when the operating environment changes.